Zirconocene/aluminoxane catalysts are known in the art for producing polymers from olefins. European Pat. No. 128,045 discloses the use of titanocenes, zirconocenes, hafnocenes and vanadocenes in combination with aluminoxane to produce polyethylene. In co-pending application Ser. No. 896,700, filed Aug. 15, 1986, is disclosed the use of certain zirconocenes and hafnocenes in combination with aluminoxane to produce dimers of alpha-olefins. In U.S. Pat. No. 4,404,344, issued Sept. 13, 1983 is disclosed the use of zirconocenes in combination with aluminoxanes to produce polymers. One problem with these zirconocene/aluminoxane catalysts is that they must be used relatively quickly after preparation or their performance as catalysts will suffer. A method has been found to stabilize these catalysts against aging during storage prior use.